1. Field of Invention
This invention relates generally to electromagnetic flowmeters having capacitive type metering electrodes, and more particularly to a flowmeter of this type which is excited by a composite electromagnetic field constituted by a DC generated magnetic comparison field and an AC generated magnetic switched field.
2. Status of Prior Art
Electromagnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104 and 3,824,856 are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because an instrument of this type is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the measuring or metering electrodes are located at diametrically-opposed positions. The operating principles of this meter are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
In the Appel U.S. Pat. No. 4,019,386 and in the Schmoock U.S. Pat. No. 4,098,118, instead of measuring electrodes in direct contact with the fluid, use is made of a pair of measuring electrodes isolated from the fluid by a layer of insulation. Each measuring electrode forms one plate of a capacitor whose dielectric is the insulation layer and whose other plate is the fluid, the electrodes acting as a capacitance sensor to detect the voltage induced in the fluid. The flow signal is thereafter measured through a pair of capacitors, the smaller the capacitance values of the capacitors, the higher is the resultant electrical impedance.
A capacitance electrode sensor of this type obviates slurry and galvanic noise problems and is not subject to leakage. Also among the advantages of a capacitance sensor over contact electrodes in a magnetic flowmeter are that the conductivity range of the fluid to be metered 20 may extent down to as low as 0.1 .mu.S/cm or less, and one may use ordinary metals for the electrodes rather than special materials capable of withstanding the adverse effects of corrosive or abrasive fluids in contact with the electrodes.
With a flowmeter of the capacitive type, in order to measure flow rate with zero stability, it is advantageous to drive the electromagnet coils with a low-frequency excitation voltage to establish the required electromagnetic field. Then virtually no disturbing induction currents are produced. And by choosing an appropriate excitation frequency having a predetermined relationship to the line frequency of the power source, this makes it possible to suppress line-frequency interference.
In a capacitive flowmeter, the signal from the capacitive electrodes of the primary are applied to the input amplifier of the flowmeter secondary. The use of a low-frequency excitation in this context is disadvantageous in regard to picking up the signal from the electrodes. The high impedance of the capacitive electrodes imposes very high requirements on the input impedance of the amplifier. In order to avoid a signal loss, the input resistance of the amplifier together with the coupling capacitance of the electrodes form a high-pass filter to the conductive medium represented by the fluid being metered.